The invention relates to an optical polarization converter device of the integrated optics structure type, in particular a wide band converter.
The device finds its application in numerous fields in which polarization controllers are required for forming amplitude modulators, circuits whose operation is independent of the incident polarization, etc.
It is known, in a guide structure allowing propagation of two separate guided modes, that an exchange of energy between the two modes may be achieved.
A guide structure coming into this category may be either a truly bimodal structure or a structure comprising two monomode guides formed on a common substrate, very close to one another, and between which the energy exchanges may take place if certain conditions are provided.
It is also known that the two modes are characterized by their propagation constants, called .beta..sub.M1 and .beta..sub.M2 in what follows.
If an energy exchange between the two modes is to be provided, it is necessary to create a disturbance introducing coupling between the two modes.
The two modes are, for example, the transverse electric TE and transverse magnetic TM modes.
The prior art circuits comprise a substrate whose orientation is such that the intrinsic birefringents between the TE/TM modes requires a periodic interaction. In fact, when a coupler is formed, i.e. a converter between two modes M.sub.1 and M.sub.2, in order to obtain an efficient interaction it is necessary for the so called "phase concord" condition to be achieved.
In general, the propagation constants .beta.M.sub.1 and .beta.M.sub.2 of the two modes are different and, in order to confirm this condition, it is necessary to have recourse to an interaction of the periodic type obeying the relationship: EQU .beta..sub.M1 -.beta..sub.M2 =(2.pi./.lambda.) (1)
in which relationship .lambda. is said period.
The region in which the coupling takes place behaves then like a coupling "network". This "network" may be either stationary or moving.
The periodicity of the artificial "network" thus created compensates therefore for the birefringence between the two modes M.sub.1 and M.sub.2.
It is clear that the greater .beta..sub.M1 is different from .beta..sub.M2, the smaller will be the period .lambda. required for phase concord. In the prior art configurations, the light propagation is perpendicular to the optical axis C of the crystal. The period .lambda. is of the order of 10 .mu.m for waves situated in the near infrared. Since the interaction lengths required are of the order of a centimeter, the number of periods is therefore high, of the order of 10.sup.3, which means that the circuits obtained have a very narrow optical band. The ratio .DELTA..lambda./.lambda. is of the order of 10.sup.-3. This results in constructional difficulties if it is desired to obtain operation at a well defined wave length. These difficulties are for example due to the effects of temperature on the optical constants of the materials, to the variations of the wave length of the sources used and to the difficulty of obtaining sufficient manufacturing information.
The aim of the invention is to overcome the disadvantages of the prior art by providing a converter with very wide optical band.
In an additional advantageous variant of the device of the invention, the band width may be predetermined accurately at the manufacturing stage.